PROJECT SUMMARY Lassa virus (LASV), the causative agent of Lassa fever, is a persistent global public health threat that has infected and killed more people than all Ebola outbreaks combined. Unlike Ebola and other viral hemorrhagic fevers (VHF), Lassa fever is perennial and endemic in West Africa resulting in approximately 300,000 infections and 5,000 deaths each year, principally in Liberia, Sierra Leone, and Guinea. The World Health Organization (WHO) reports that the incidence of Lassa fever is increasing, perhaps as a consequence of climate change, as is the severity and case fatality rate of the disease. As LASV has been imported into non-endemic countries ? with more than 32 cases reported, one third of which were fatal ? the significance of enhanced detection and management of Lassa fever extends beyond West Africa. Many of these deaths could be prevented with better diagnostics, supportive clinical care, and therapeutics. Yet despite being a major cause of death in West Africa, Lassa fever remains under-diagnosed, understudied, and largely ignored. For these reasons, in 2016, the WHO released a research and development blueprint call-to-action, which identified LASV as a ?top priority emerging pathogen? that is likely to cause a severe outbreak in the near future and urgently needs to be studied. Civil unrest coupled with inadequate healthcare and research infrastructure in West Africa have prevented clinical research on this high-priority pathogen, thereby limiting our understanding of the true burden of Lassa fever, the pathogenic mechanisms, and the infectivity of survivors. The primary goal of this proposal is to leverage clinical infrastructure established during the Ebola epidemic to fill critical gaps in our understanding of Lassa fever. Specifically: ? In AIM I we will establish molecular diagnostics at Phebe Hospital in Bong county, Liberia ? a hyper-endemic area for Lassa fever to determine the prevalence of acute Lassa fever among admitted febrile patients as well as the seroprevalance of prior LASV exposure. ? In AIM II we will probe putative pathogenic mechanisms of acute and convalescent Lassa fever including immune activation, endothelial dysfunction, and persistence of viral antigens in genital compartments. ? In AIM III we will characterize the compartmental dynamics of LASV in blood, semen and cervical-vaginal fluid to determine the duration of viral shedding and assess the infectivity of PCR-positive samples. We will conduct this work in the context of close and strong working relationships with healthcare leaders in West Africa and a well-developed infrastructure for clinical research we have established in Liberia where we have recruited, enrolled, and longitudinally followed and sampled over 300 Ebola survivors. Establishment of these cohorts allowed our team to determine the feasibility of our approach and develop, pilot, and refine the procedures needed to sustain high-quality data collection during the study of VHFs. This work also allows us to build and support capacity. Our team in Liberia has been trained in research methodology and is being encouraged to develop their own proposals. We have also established a state-of-the-art molecular diagnostic lab and validated a diagnostic platform for the detection of Ebola RNA in blood, semen, and vaginal fluid at Phebe Hospital and have trained this site in quality control. This experience will be applied to the study of Lassa fever. Collectively, the proposed work will provide a much-needed characterization of Lassa fever epidemiology, pathogenesis, and compartmental viral dynamics and will provide a wellspring of data that will inform public policy and individual care. Furthermore, the re-establishment of a Lassa fever center in one of the hardest hit areas will facilitate on-going research to advance the management and prevention of this deadly infectious disease. While much about Lassa fever remains understudied, it is clear that each year this infection will continue to kill more people than all other hemorrhagic viruses combined. What we propose is a series of studies that are long overdue.